Greenhouses or other agricultural structures, such as agricultural tunnels, may be covered with flexible films comprising polymeric materials that are tailored to provide a favorable environment for the cultivation of crops and plants. The covers are employed to provide protection for the crops grown underneath from adverse weather conditions and to create a greenhouse effect by selectively filtering solar radiations.
The composition of the polymeric film is important and, in particular, should provide the film with: adequate mechanical properties, including resistance to tears, film elongation and stresses from atmospheric agents; high levels of light transmittance, especially in the region of chlorophyll photosynthesis; capacity to absorb radiation in the far infrared region of the spectrum, i.e. from 7000 to 13000 nm; and good light stability for an extended lifetime.
To obtain better thermal insulation during the night and reduce energy consumption, such greenhouses may use several separate films to create a layered structure that is subsequently inflated. For example, the greenhouses are covered with two, or rarely three, separate films which are individually installed one above the other and then air is inserted between them to keep them apart. This practice is thought to achieve better thermal insulation and also help in the case of strong winds and snow, as the air between the layers helps to keep the structure well tensioned and prevents any mechanical fatigue that would result from uncontrolled movement in the wind. The lifetime of plastic films on inflated greenhouses is also improved compared to single film uninflated greenhouses.
The covering for inflated greenhouses is achieved with two or rarely three films, exhibiting the same or different characteristics. The films are laid on the greenhouse structure and fixed with special clips or profiles, one after the other. In some geographical regions a single tube is laid on the structure and fixed. In both cases, after fixing the films or tube, air is inserted to keep the films or the two sides of the tube apart.
In practice, the installation process is not easy because the films must be unrolled and unfolded at heights of from 4 to 6 meters from the ground and on long lengths, usually from 50 to 100 meters, and then fixed at the sides of the span throughout their length. Afterwards, the same process has to be repeated for the second film and so on. If during the installation there are winds, there is a danger of the films being torn and for the installers to be injured. Due to these reasons and also due to the time it takes to install the two (or more) films, the process is rather expensive.
Using tubes also has disadvantages because the upper and lower film cannot be different, as required by best practices (e.g. a non-thermic mechanically strong upper film and a thermic film with anti-dripping characteristics on the lower one), and also because there are limitations on the width of the span (tubes are typically less than 8 meters), because greenhouse film tubes are currently made with a circumference of up to 16 meters at most.
WO97/00006 describes a greenhouse film having variable light diffusion properties. In one embodiment, the film may be combined with a separate, clear low density polyethylene film to produce a double-skin cover capable of being attached to a greenhouse structure and inflated with air.
Accordingly, there is still a need for an improved polymeric film that can be used in association with agricultural structures, such as greenhouses, agricultural tunnels, animal husbandry buildings, plant expositions etc. There is also a need for an inflatable film exhibiting excellent mechanical and optical properties that can be installed in a faster and simpler way, to a variety of shaped greenhouses.